This application claims the benefit of Korean Patent Application No. 1998-42836, filed on Oct. 13, 1998, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a bidirectional optical amplifying apparatus in a bidirectional WDM (wavelength division multiplexing) optical communication network, more particularly to a bidirectional optical amplifying apparatus which uniformly controls optical gain and gain flatness of a WDM channel in a bidirectional optical communication network that allows bidirectional communication using a single mode optical fiber (SMF).
2. Discussion of the Related Art
In general, WDM has been developed for increasing the channel capacity of an optical fiber. A WDM system adopts a plurality of optical signal channels, and a particular wavelength is allocated to each channel. Because each channel may transmit a plurality of information sources by TDM (Time Division Multiplexing), added optical channels increase their capacity in proportion to channels of single channel system. In the transmission end of the WDM system, a number of single channels are created. The single channels are multiplexed as WDM optical signals, then the multipexed WDM optical signals are transmitted through an optical line. At the receiving end, the WDM optical signals are demultiplexed to each channel in order to be transmitted through designated receiver paths. The multiplexed WDM optical signal may be directly amplified through an optical amplifier, for example a doped optical amplifier, at the same time. Such an optical amplifier is very useful for a WDM system in a long distance optical system.
A bidirectional WDM optical communication system is used for transmitting or receiving a plurality of optical signals via an optical fiber. Furthermore, the optical amplifier is a most important element adopted in the bidirectional WDM optical communication system.
FIG. 1 is a schematic diagram showing a general bidirectional WDM optical transmission system.
Referring to FIG. 1, a bidirectional optical amplifier 140 amplifies and transmits channel signals respectively of the forward transmission path 161, which transmits signal from left optical line 151 to right optical line 152, and the reverse or backward transmission path 162, which transmits signals from right optical line 152 to left optical line 151. Wavelengths for the forward transmission and the reverse or backward transmission are different from each other.
First (forward) optical sending-end 111 comprises a transmitter Tx1 employing wavelength xcex1, and a transmitter Tx2 employing wavelength xcex2. First optical receiving-end 112 for receiving signal from the first (forward) optical sending-end 111 comprises a receiver Rx1 employing the wavelength xcex1, and a receiver Rx2 employing the wavelength xcex2.
Second (reverse or backward) sending-end 121 comprises a transmitter Tx3 employing wavelength xcex3, and a transmitter Tx4 employing wavelength xcex4. Second optical receiving-end 122 for receiving signal from the second (reverse or backward) optical sending-end 121 comprises a receiver Rx3 employing the wavelength xcex3, and a receiver Rx4 employing the wavelength xcex4.
First optical multiplexer 131 multiplexes optical output signals of the first (forward) optical sending-end 111, and transmits the multiplexed optical output signal to the first optical receiving-end 112. The first optical multiplexer 131 demultiplexes an optical output signal of the second (reverse or backward) optical sending-end 121 and transmits the demultiplexed optical output signal to the second optical receiving-end 122.
Second optical multiplexer 132 multiplexes the optical output signal of the second (reverse or backward) optical sending-end 121 and transmits the multiplexed optical output signal to the second optical receiving-end 122. The second optical multiplexer 132 demultiplexes the optical output signal of the first (forward) optical sending-end 111 and transmits the demultiplexed optical output signal to first optical receiving-end 112.
The bidirectional optical amplifier 140 should minimize any reduction of transmission quality caused by back reflections. To configure a bidirectional amplifying apparatus for minimizing reduction of transmission quality, the following method has been suggested. The wavelength(s) of WDM optical signal, which travels forward, and the wavelength(s) of WDM optical signal, which travels in a reverse or backward direction, are allocated with different wavelengths, respectively. And, an optical filter is provided to prevent the back reflected optical signal from passing through the optical amplifier.
FIG. 2 is a schematic diagram showing a bidirectional optical amplifying apparatus in which back reflections are removed, in a conventional optical transmission system. The bidirectional optical amplifying apparatus is comprised of optical connectors 211, 251, bidirectional optical amplifier 221, 222, circulators 231, 232 and optical filters 241, 242.
The forward WDM transmission channel is comprised of the first optical connector 211, the first bidirectional optical amplifier 221, the first circulator 231, first optical filter 241, the second circulator 232, the second bidirectional optical amplifier 222, and the second optical connector 251. The reverse WDM transmission channel is comprised of the second optical connector 251, the second bidirectional optical amplifier 222, the second circulator 232, the second optical filter 242, the first circulator 231, the first bidirectional optical amplifier 221, and the first optical connector 211. Here, characteristics of the first bidirectional optical amplifier 221 and the second bidirectional optical amplifier 222 are the same.
The optical connectors 211, 251 are elements used for transmitting output signals of the bidirectional optical amplifiers 221, 222 to the optical line. The optical connectors 211, 251 have the characteristic of reflecting back some of the optical signals by forming a reflective surface at a node.
The bidirectional optical amplifiers 221, 222 are comprised of passive elements and a pump laser. The bidirectional optical amplifiers 221, 222 are designed to be capable of bidirectional amplification by removing an isolator in a conventional unidirectional optical amplifier.
The circulators 231, 232 are elements in which a unique output port is provided according to the input direction of an optical signal. For example, the circulator has the characteristic that an input signal at port 1 of the circulator should exit through port 2, and an input signal at port 3 of the circulator should exit through port 1. The other words, the output for each input port is the first port reached by rotating the ports of the circulator clockwise.
The bidirectional optical amplifying apparatus in FIG. 2 operates as follows. An input signal (F_WDM_IN) in the forward WDM transmission line is represented with a dotted line in an upper part of the FIG. 2. An input signal (R_WDM_IN) in the reverse WDM transmission line is represented with a dotted line in a lower part of the FIG. 2. The wavelengths of the two input signals (F_WDM_IN and R_WDM_IN) are different from each other.
The forward WDM optical input signal (F_WDM_IN) is amplified in the first bidirectional optical amplifier 221. The F_WDM_IN signal passes through the first optical circulator 231, the first optical filter 241, the second circulator 232, and the second bidirectional optical amplifier 222, and is output as a forward WDM optical output signal (F_WDM_OUT).
The reverse WDM optical input signal (R_WDM_IN) is amplified in the second bidirectional optical amplifier 222. The R_WDM_IN signal passes through the second circulator 232, the second optical filter 242, the first circulator 231, and the first bidirectional optical amplifier 221, and is output as a reverse WDM optical output signal (B_WDM_OUT).
Here, the forward WDM optical output signal (F_WDM_OUT) is back reflected by the second optical connector 251 so that the back reflected WDM optical signal (BACK_REF_WDM) is produced. The BACK_REF_WDM signal passes through the second bidirectional optical amplifier 222 and travels through the second circulator 232 to the second optical filter 242. The second optical filter 242 is designed to pass the reverse signal and cut off the forward signal. Therefore, the second optical filter 242 filters the BACK_REF_WDM such that it does not pass through the filter to the first bidirectional optical amplifier 221.
Also, the reverse WDM optical output signal (not shown) is back reflected by the first optical connector 211 so that a back reflected WDM optical signal is produced. Although it passes through the first bidirectional optical amplifier 221 and travels through the first circulator 231 to the first optical filter 241, the back reflected WDM optical signal is cut off by the first optical filter 241.
In this way, the back reflected optical signal in the bidirectional optical transmission is cut off by the filter, thereby preventing a reduction in efficiency caused by back reflections.
But, the conventional bidirectional optical amplifying apparatus in FIG. 2 has disadvantage in that the amplifying apparatus cannot overcome a variation of optical gain which occurs in the bidirectional optical amplifier when coupling/dividing a WDM channel. Such a situation is illustrated with reference to FIG. 3.
FIG. 3A is a graph showing an input-output gain of the bidirectional optical amplifier of FIG. 2, in a normal state. FIG. 3B is a graph showing an input-output gain of the bidirectional optical amplifier in FIG. 2 when the channel is branched off. We consider a case where the gain of the bidirectional optical amplifier is 20 dB, and the input signal has four (4) forward and four (4) reverse channels having different wavelengths, respectively.
Referring to FIG. 3A, four channels xcex1, xcex2, xcex3, xcex4 of a forward WDM optical input signal 311, and four channels xcex5, xcex6, xcex7, xcex8 of a reverse WDM optical input signal 321 have the same optical intensity (for example, xe2x88x9210 dBm). When the input signal of each direction is input to the bidirectional optical amplifier, each channel of the forward WDM optical output signal 312 and reverse WDM optical output signal 322 maintains the same intensity (for example, +10 dBm) in a normal state, as shown in FIG. 3A.
FIG. 3B shows the case where the second and third channels xcex2, xcex3 of a forward. WDM optical input signal 331 and sixth and seventh channels xcex6, xcex7 of a reverse WDM optical input signal 341 are branched off. The optical intensity of each channel xcex1, xcex4 of the forward WDM optical output signal 332 and each channel xcex5, xcex8 of the reverse WDM optical output signal 342 increases non-linearly (for example, exceeds +10 dBm).
So when a WDM channel is branched off, the optical output intensity of the other WDM channels increases abnormally. Such an increase of optical intensity introduces a nonlinear effect; and the difference of the optical intensity makes the transmission quality of each channel different. This is one of the factors making network management difficult.
On the other hand, when the WDM channel is coupled, the optical output intensity of the other WDM channels decreases and the transmission quality falls off.
Accordingly, it is required to develop a bidirectional optical amplifier for maintaining the optical gain of the other WDM channel within a certain range, maintaining the gain flatness of each channel, although coupling/branching of other WDM channels occurs.
Accordingly, the present invention is directed to a bidirectional optical amplifying apparatus and optical gain controlling method in a bidirectional WDM optical communication network that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to control the energy source of an optical amplifier according to the variation of a WDM channel transmitted through a bidirectional optical amplifier in a bidirectional WDM optical communication network. The present invention provides a bidirectional optical amplifying apparatus and an optical gain controlling method in the bidirectional WDM optical communication network which maintains optical gain and gain flatness of respective channels in the bidirectional optical amplifier uniformly, in spite of coupling and branching off of one or more channels.
In one embodiment, the present invention provides an optical amplifying apparatus amplifying an input optical signal to a predetermined level according to the optical output of a pump laser, and outputting the amplified optical signal in a bidirectional WDM (wavelength division multiplexing) optical communication network, the optical amplifying apparatus comprising:
control means for controlling the optical output of the pump laser according to the variation of WDM channel transmitted through the bidirectional optical communication network, and uniformly maintaining the transmission gain of the bidirectional optical communication network.
In another embodiment, the present invention provides an optical amplifying apparatus for amplifying an optical signal of a WDM channel input from a bidirectional WDM optical communication network in two directions to a predetermined level according to an optical output of a pump laser, and outputting the amplified optical signal in a bidirectional WDM optical communication network, the optical amplifying apparatus comprising:
control means for detecting the optical signal, transmitted through the bidirectional optical communication network, determining a variation of the WDM channel (i.e., through branching or coupling of one or more WDM channels), controlling the optical output according to the determination, and maintaining a transmission gain of the bidirectional optical communication network, uniformly.
In yet another embodiment, the present invention provides an optical gain controlling method of a optical amplifying apparatus amplifying an optical signal of a WDM channel to a predetermined level according to an optical output of a pump laser and outputting the amplified optical signal in a WDM optical communication network, comprising the steps of:
i) detecting an input and output WDM optical signal in the optical amplifying apparatus; and
ii) compensating gain error of the optical amplifying apparatus caused by a WDM channel variation, using the WDM optical signal input and the WDM optical signal output.
In a further embodiment, the present invention provides a method for controlling the optical gain of a WDM optical amplifying apparatus amplifying an optical signal of a WDM channel to a predetermined level according to an optical output of a pump laser and outputting the amplified optical signal in a bidirectional WDM optical communication network, comprising the steps of:
detecting a forward WDM optical signal and a backward or reverse WDM optical signal input and output in two directions, respectively; and
compensating the gain error of the bidirectional optical amplifying apparatus caused by a WDM channel variation, using the detected bidirectional WDM optical signal.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.